A method for measuring the material concentration C0 with a chemical sensor is based on a measurement equation of the response signal S0:S=f(C0;k0,k1 . . . kp−1)  (1)Wherein, the parameters kj depend on the structural properties of the sensor, the composition of the sample, and the temperature, pressure, and flow state of the sample and environment. The current signal S of the most commonly used electrochemical, semiconductor, and catalyst-based sensor usually meets the following measurement equation:S=kC0+k0  (2)Wherein, parameters k0 and k are zero point and sensitivity parameter of a sensor, respectively.
In the actual application of a sensor, the response signal of the sensor may be affected by factors such as the gas flow rate, pressure, temperature, and humidity, and other gas components, and the sensitivity of the sensor may vary owing to ageing, inactivation, activation, or poisoning, etc. Therefore, a sensor usually has to be calibrated with at least two standard samples with known concentration under conditions close to the actual application conditions, including gas flow rate, pressure, temperature, humidity, and gas components before it can be used, in order to ascertain the applicability of the equation (1) and the parameters kj of the sensor; in addition, the time of calibration should be close to the time of measurement as far as possible, to avoid the interferences described above.
In actual application, the calibration process involves a series of problems, for example, there are technical and safety risks, since it is difficult to prepare, store, carry or use calibration samples with low concentration, high volatility, high reactivity, and high hazards (toxic, harmful, flammable, and explosive); there are risks related with reliability and effectiveness, since it is usually difficult to simulate the actual situation with the calibration sample and calibration conditions; even though the actual situation can be simulated in the calibration process, it is difficult to carry out calibration at the site of application; even though the calibration can be carried out on site, many user usually neglect calibration.
For example, it is difficult to obtain or use calibration samples of organic volatile materials, such as formaldehyde and benzene, etc. It is difficult to carry out calibration on site even in the industrial and environmental safety monitoring field where on-site calibration is applied most widely, especially at coal and oil gathering and transportation sites. Safety accidents incurred by misreport or concealment often occur owing to negligence or improper calibration. Especially, in the civilian field, such as blood glucose test and indoor gas leakage warning, etc., in household application, calibration is seldom carried out, and the resulting accidents are often reported.
At present, efforts made to solve that problem mainly involve providing safe, convenient, and reliable automatic calibrators. For example, in recent years, Honeywell disclosed a number of patents related with sensor calibration and automatic calibration methods (U.S. Pat. No. 7,975,525B2, U.S. Pat. No. 7,661,290B2, US2006/0266097A1, US2005/0262924A1, U.S. Pat. No. 7,401,493B2, U.S. Pat. No. 7,581,425B2, U.S. Pat. No. 7,655,186B2, U.S. Pat. No. 7,071,386B2, U.S. Pat. No. 6,918,281, US2006/0042351A1), and Drager has disclosed several patents related with sensor calibration (U.S. Pat. No. 7,704,356B2, U.S. Pat. No. 7,645,362B2) lately a common characteristic of these patents is that a standard gas is required, while only the method for producing the standard gas varies among these patents. Is there any calibration method that doesn't require a standard gas?
In 1987 and 1989, City Tech and Drager, respectively, disclosed patents (U.S. Pat. No. 4,829,809, U.S. Pat. No. 4,833,909) in which calibration could be carried out without a calibration sample, an electrochemical sensor is placed in an enclosed container filled with a sample, the gas concentration is ascertained through coulomb electrolysis after the tested material is completely electrolyzed, and thereby the sensor is calibrated.
In 2000, Industrial Scientific discloses a patent (U.S. Pat. No. 6,055,840), in which a method for determining the gas concentration by regulating and controlling the resistance in a gas diffusion channel quantitatively was described. However, that method is also inconvenient to use in actual application, because the diffusion coefficient of the tested gas and at least one physical dimension of a gas diffusion channel must be known.
These methods still belong to methods for laboratory research or analysis, and are difficult to use in actual application. At present, the calibration of gas sensors still rely on a calibration method in which materials with standard concentration provided by a standard measurement department are used.